Systems for providing either electrical stimulation of the brain or coupling as fluid to or from the brain are coming into increased use for various purposes. Electrical stimulation of the brain is utilized for relief of chronic pain and treatment of movement disorders. A typical electrical brain stimulation system comprises a pulse generator operatively connected to the brain by a lead. The lead has one or more electrodes at its distal end, designed to be implanted within the patient's brain at a precise location, so that the electrode or electrodes are optimally and safely positioned for the desired stimulation. The lead is connected to the pulse generator at its proximal end, and also needs to be anchored with respect to a burr hole drilled in the patient's skull or cranium, in order to reliably secure the electrodes at the target location. Likewise, in the case of a catheter for providing fluid to the brain or for providing drainage, it is necessary to be able to secure the distal portion of the catheter that passes through the skull and transfers the fluid at a predetermined exact location within the brain. Still further, for a combined catheter and lead member, such secure and reliable anchoring of the member so that the distal end is precisely located within the skull, is very important. As used herein, the term lead, or lead-type member, refers to any such cranial catheter or lead.
Reference is made to U.S. Pat. No. 5,464,446, "Brain Lead Anchoring System," assigned to Medtronic, Inc., which is incorporated herein by reference. The referenced patent illustrates an effective lead anchoring system, and it discusses the method of providing access through the skull by drilling a burr hole with a cranial drill, inserting a stimulation lead through the burr hole and positioning it so that the electrode or electrodes are at the desired stimulation site. The lead is positioned using a stereotactic instrument, which permits a very precise movement within the brain. Once the lead is positioned and tested to determine that the results of stimulation are satisfactory, it is critical that the lead is not moved, since even a small displacement can result in less than optimal results, and even injury to the brain. Accuracy should be maintained with .+-.0.5 mm.
The anchoring system of the '446 patent shows a basic anchor for fixing the lead in place with the distal portion extended through the cranial burr hole, and then securing it by bending it into a slit such that it is held by a friction fit. However, the lead must first be removed from the stereotactic instrument, such that this system does not provide a reliable way for accurately securing the lead, or catheter, before it is bent into the fixation position. Thus, such a system does not reliably preclude a small movement of the distal end of the lead at the time of fixating, or securing the lead in place. Rather, it is required that the lead be removed from the stereotactic device before the lead can be fixed to the skull.
In U.S. application Ser. No. 08/705,566, filed Aug. 29, 1996, assigned to the same assignee as this invention, there is shown an apparatus and method wherein a compression screw cap is screwed down onto a compressible seal, the seal being flexible and compressed laterally against the outer wall of the lead. This system provides a substantial improvement over the prior art, and in particular enables securing the lead with respect to the skull before it is stereotactically released. However, it is operative only when the lead is to be positioned in the center of the burr hole. If, however, the target localization procedure indicates that the lead must be inserted off-center, this assembly and the procedure of using it is not adequate; the lead must be bent in order to fix it through the iso-centric burr hole cap, and this bending results in a lead displacement from the stereotactically determined target. There thus remains a need in the art for a reliable system which enables off-center, or eccentric placement of the lead through the burr hole, without requiring stereotactic release of the lead.